Heat pump devices exist that supply a high-frequency low voltage to a compressor during a shutdown during heating in order to improve the rising speed of the air conditioner when heating is started (for example, see Patent Literature 1). A similar technique is used in a heat pump device that supplies a single-phase AC voltage having a higher frequency than that at the time of a normal operation to a compressor when it is detected that the temperature of the air conditioner's surroundings becomes low (for example, see Patent Literature 2).
Moreover, in order to prevent the refrigerant retention phenomenon from occurring, a heat pump device exists that generates, as drive signals for a compressor motor, signals to be output with a predetermined static phase angle in the PWM output in a two-phase modulation system during the restricted energization for preheating the compressor (for example, see Patent Literature 3).
Patent Literature
Patent Literature 1: Japanese Unexamined Utility Model Registration Application Publication No. S60-68341
Patent Literature 2: Japanese Patent Application Laid-Open No. S61-91445
Patent Literature 3: Japanese Patent Application Laid-Open No. 2007-166766
The above Patent Literatures 1 and 2 disclose a technique facilitating a lubricating action in the compressor by heating the compressor or keeping the compressor warm by applying a high-frequency AC voltage to the compressor in response to a decrease in outside air temperature.
However, there is no detailed description in Patent Literature 1 of the high-frequency low voltage, and the output change, which depends on the stop position of the rotor, is not taken into consideration. Therefore, there is a problem in that the desired amount of heat for the compressor may not be obtained.
In contrast, there is a description in the above Patent Literature 2 of an application of a voltage from a high-frequency (e.g., 25 kHz) single-phase AC power supply and the effects, such as noise reduction due to being outside the audible range, vibration suppression due to not being the resonance frequency, input reduction and prevention of temperature increase due to the reduction in current by the amount of inductance in the winding, and rotation suppression of the rotating part of the compressor.
However, in the technique in Patent Literature 2, because a high-frequency single-phase AC power supply is used, a fully-off period, during which all the switching elements are off, is generated for a relatively long time as shown in FIG. 3 in Patent Literature 2. At this point, a high-frequency current is regenerated to the DC power supply without it flowing back to the motor via the freewheeling diodes and the current decays fast during the off-period; therefore, there is a problem in that a high-frequency current does not efficiently flow to the motor and thus the heating efficiency of the compressor degrades. Moreover, when a small motor having low iron loss is used, the amount of heat generation becomes small with respect to the applied voltage; therefore, there is a problem in that the necessary amount of heat cannot be obtained with a voltage that is within the usable range.
Moreover, Patent Literature 3 discloses a technique of performing preheating such that the rotor does not rotate by performing restricted energization in which a DC current is caused to flow in the motor winding.
However, the winding resistance of a motor tends to decrease due to the highly efficient design of recent motors. Therefore, in the case of the preheating method of causing a DC current to flow in the motor winding as described in Patent Literature 3, because the amount of heat generation is given by the product of the winding resistance and the square of the current, the current is increased by the amount of reduction of the winding resistance. Consequently, a problem arises with the heat generation due to the increase of the inverter loss and also other problems arise such as a decrease in reliability and an increase in the cost of heat dissipation structures.